The present invention relates to an electroluminescence (EL) display using a metal electrode having a reflection scattering property, in particular, for preventing a decrease in contrast due to reflection of ambient light.
Recently, development of illumination devices having electroluminescence elements (hereinafter referred to as “EL display”) is in progress. As shown in FIG. 5, an EL display 51 comprises a transparent substrate 56 and an EL element 57. The EL element 57 is formed from a laminate structure in which a organic thin layer 53 is provided. The organic thin layer 53 which includes at least a light emitting layer provided with blue, green and red light emitting regions and which is sandwiched between a metal electrode 52 with a small work function and a transparent electrode 54. Electrons are injected into the organic thin layer 53 from one of the electrodes and holes are injected from the other electrode. Light is emitted when the electrons and holes recombine within the light emitting material layer. The emitted light is visible from the transparent substrate 56 side.
Luminance of the display is often enhanced by reflecting the light rays from the metal electrode 52 in the EL device 57, by using metals having high reflectivity, such as magnesium (Mg), magnesium-silver alloy (MgAg), magnesium-indium alloy (MgIn), aluminum (Al), lithium-aluminum alloy (LiAl), for the metal electrode 52. Accordingly, in the device structure described above, the metal electrodes 52 can function as a mirror with a high light reflecting property when the device is not generating light by electroluminescence, which can project the ambient environmental view, or can cause problems in the application of the display. The problems are for example a decrease in contrast due to reflection of ambient light, and failing to display the color of black.
In order to solve the above described requirements, Japanese Laid-Open Patent Publication 9-127885 discloses an organic electroluminescence device in which a ¼ wavelength plate and a linear polarizer are disposed in that order, on the transparent substrate side, which functions as the light emitting surface.
However in this organic EL device, the light emitted from the light emitting layer is also transmitted through the polarizer, so that the light attenuates during the transmission therethrough and decreases the amount of light emitted.